This invention relates to an image forming apparatus which develops image by means of liquid developer.
A conventional image forming apparatus uses insulating liquid developer. The insulating liquid developer is made by dispersing toner particles in insulating liquid such as aliphatic saturated hydrocarbon. In an image developing process, electrostatic latent image is formed on an image bearing member (such as a dielectric member). The insulating liquid developer contacts with the image bearing member. Then, the toner particles in the insulating liquid developer m-grate to electrostatic latent image on the image bearing member, due to electrical migration. Consequently, the toner particles selectively adhere to the electrostatic latent image on the image bearing member.
Compared with an image forming apparatus using dry toner powders, the image forming apparatus using the liquid developer has an advantage that the toner particles in the liquid developer are smaller than dry toner powders, which is suitable for producing image of a high resolution. Further, the toner particles in the liquid developer are fixed to a recording media with less energy, compared with dry toner powders.
However, due to a smell of aliphatic saturated hydrocarbon, the image forming apparatus using the insulating liquid developer is not suitable for the use in office and home. On the other hand, if aquatic liquid developer is used in the above-mentioned image forming apparatus, the electrostatic latent image on the image bearing member may disappear when the aquatic liquid contacts the image bearing member (because of the conductivity of the aquatic liquid).
Japanese Patent Publication No. 54-25418 discloses an image forming apparatus using aquatic liquid developer. In such an image forming apparatus, the image bearing member and the aquatic liquid developer are faced with each other, with a minute gap provided therebetween. With this, the aquatic liquid developer flies across the gap and adhere to electrostatic latent image on the image bearing member.
In such an image forming apparatus, it is necessary to control the minute gap between the surface of the aquatic liquid developer and the image bearing member, for obtaining image of a high resolution. However, such a controlling of the minute gap is difficult.
In order to solve this problem, it is studied to provide a mesh developer retainer for holding the liquid developer (Japanese Patent Application No. HEI 9-234258). The developer retainer has a number of fine holes in which the liquid developer is held.
However, in such a mesh developer retainer, it is still difficult to control the gap between the surface of the liquid developer and the image bearing member. That is, if the developer retainer is made of material which repels the liquid developer, the liquid developer does not easily flows in the fine holes. Conversely, if the developer retainer is made of material which is wettable to the liquid developer, the liquid developer tends to flow through the fine holes to the image bearing member.
Additionally, in the mesh developer retainer, the surface of the liquid developer held in each fine hole is concave-shaped due to the surface tension. That is, a center of the surface of the liquid developer in each fine hole is o shifted away from the image bearing member. With this, the density of the electric flux lines (generated by the electrostatic latent image on the image bearing member) is dispersed throughout the surface of the liquid developer in each fine hole. This weakens the electrostatic force acted on the liquid developer, which prevents the development of image of a high resolution.